The present invention relates to an inductive sensor for measuring a current in a conductor, of the kind including a generally annular winding delimiting a passage for the conductor. The winding extends homogeneously over most of the circumference of a ring, except for a section where the winding has a structural discontinuity.
Placing a coil with contiguous turns closed on itself and constituting a wound ring around a conductor to measure the current flowing in it is known in the art. A coil of the above kind is referred to as a Rogowski coil.
When an alternating current flows inside the conductor, a voltage is induced at the terminals of the coil. The voltage is proportional to the time derivative of the current flowing in the conductor. To evaluate that current the terminals of the coil are connected to an integrator circuit known in the art.
A sensor of the above kind is routinely used on the input and/or output busbars of a three-phase transformer.
In installations known in the art one sensor is provided on each of the four busbars of the three-phase supply. Each sensor includes a single winding carried by a non-magnetic support of constant cross section bent on itself to form a ring. The winding has turns regularly distributed about the non-magnetic support. The non-magnetic support has a discontinuity formed by an interruption delimited between its ends which are bent towards each other.
The winding has a similar structural discontinuity in this region of the non-magnetic annular support, where the number of turns is reduced. The region in which the non-magnetic support is interrupted generally has no turns at all. That region corresponds to the ends of the winding, from which depart wires constituting terminals of the winding.
In the region where the sensors are installed, the three-phase power supply busbars are generally disposed side-by-side in a row. Each of the sensors is therefore affected by the current flowing in the adjoining busbars. In particular, because of the presence of a discontinuity in the toroidal structure if the winding, depending on the position of the sensor around a busbar, and in particular its position relative to the adjoining busbars, the voltage at its terminals can vary significantly because of the effect of the adjoining phases flowing in the adjacent busbars.
Thus, as a general rule the asymmetrical nature of the structure of the winding makes the sensor highly sensitive to external magnetic fields; its sensitivity depending in particular on the position of the sensor.
An object of the invention is to propose an inductive sensor that is relatively insensitive to interfering magnetic fields coming, in particular, from busbars in the immediate vicinity and carrying a current.
To this end, the invention provides an inductive sensor for measuring a current in a conductor, of the aforementioned kind, characterized in that it includes at least one substantially identical complementary winding, which windings are disposed coaxially and oriented so that the structural discontinuities of the windings are regularly distributed in the angular direction about the common axis of the windings, and in that the windings are connected in series.
Particular embodiments of the invention have one or more of the following features:
it includes two windings disposed symmetrically with respect to their common axis;
each winding is wound on an annular non-magnetic core;
each winding includes an even number of layers of superposed contiguous turns extending in the direction of the circumference of the ring, and the turns of the layers of the same pair of layers are wound in opposite directions;
the windings are received in a protective casing;
the protective casing delimits an annular chamber receiving the windings, which are encapsulated in a resin therein; and
each winding has along its periphery two parallel rectilinear sections connected to each other by two semicircular sections.